<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta>
    <title>圆角</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .box {
            background-color: yellow;
            width: 100px;
            height: 100px;
            margin: 30px auto 0;
            border: 1px solid black;
           
            border-top-left-radius: 50px;
            border-top-left-radius:50px 100px;
            border-top-left-radius: 50%;

            border-radius: 20px / 30px;
            border-radius: 50%;
            border-radius: 50% / 20%;
        }


    </style>
</head>
<body>
    <div class="box"></div>

</body>
</html>